a. Field of the Invention
This invention relates to turbine wheel flow measuring transducers and, more specifically, to flow meters that measure low rates of a gaseous flow.
b. Description of the Prior Art
Conventional flow tube type gas flowmeters took the form of a vertically mounted glass tube containing a spherically shaped ball float that moved upward in the glass tube in proportion to the flow rate of the gas flowing through the glass tube. Such flow tube type flowmeters have historically been employed in various gas analyzers, gas metering devices and gas chromatographs. It is considered to be an advantage to have an economically priced flow sensor that produces an electrical signal in proportion to the flow rate as a suitable alternate for glass tube flowmeters but such a device is not currently being routinely employed, so far as is known, perhaps due to the price of commercially available electronic flow sensors.
A great majority of instrument related gas flow measurement applications call for very low gas flow rates to be measured. The majority of all such flow rate requirements was within the 100 to 1000 milliliter range with almost all falling within the 20 to 10,000 milliliter/minute flow range. Accordingly, a gas flow meter having the following characteristics would be highly desirable: inexpensive to construct; simple and reliable design; capable of measuring flow rates from 20 ml/minute to 10,000 ml/minute; having a very low pressure drop across the flow transducer; linear electrical output directly proportional to the flow rates; and small in size.
Currently, commercial gas flow transducers that are popular and used in a limited sense in the instrument industry are of two principal types: thermal sensors and axial turbine flow sensors. Typical small quantity costs of these sensors are such that manufacturers of gas instruments continue to use conventional flow tube type flow meters for the great majority of applications.
The thermal method is quite old and was first described by C. C. Thomas in the JOURNAL OF THE FRANKLIN INSTITUTE, 172,411 in 1911. Then, later an improved thermal flow sensing method using matched thermistors was described by R. S. Goodyear in the publication ELECTRICAL MANUFACTURING on page 90 in October 1956. It is this matched thermistor pair design that is in current usage by commercially available thermal sensing flow transducer manufacturers. However, hand matching of the thermistors is required for proper results and costs are high.
The axial flow turbine type flow transducer (Norton in HANDBOOK OF TRANSDUCERS FOR ELECTRONIC MEASURING SYSTEMS, first published in 1969) was originally developed for aerospace flow measurements but has since become popular in numerous other fields. The typical turbine rotor resembles a propeller blade suspended inside a tube so that as a gas flow moves through the tube, the turbine rotor spins in proportion to flow rate. Bearing friction becomes a paramount problem whenever gas flows below 1000 ml/minute are to be measured. So, as sensitivity for this type of turbine gas flow meter increases, then the costs related to construction to overcome frictional problems accordingly also increased since rotor blade balancing problems were accomplished by tedious hand methods.
Other conventional flow meters have taken the form of paddle wheel designs of the classical Pelton wheel turbine class having large impact surface areas. Examples of such large impact surface area turbine wheels are disclosed in U.S. Pat. Nos. 4,030,357; 3,866,469; 3,021,170; 4,011,757; 3,867,840; 400,331; 4,172,381; 3,792,610; 3,949,606; 4,023,410; and 3,701,277. A review of this prior art has disclosed that much attention has been given to turbine wheel or paddle wheel designs wherein liquid flows are to be measured. These paddle wheel designs were typically fabricated in sensitive versions using a plastic that provided a specific gravity approximately equal to that of the liquid being measured. This was of great benefit in liquid flowmeters because even though the turbine wheel was quite heavy, it floated in the liquid being measured, thereby removing the weight of the turbine wheel from its bearings and substantially reducing friction problems. Since all liquids are quite viscous in comparison to gases at usual conditions, Pelton wheel turbines could have only so many paddles or the liquid being measured could be so viscous that an unacceptable drag upon the turbine wheel was produced at high flow rates, causing the sensor to produce a non-linear electrical output.
The prior art turbine wheels tended to float in a liquid being measured and thereby removed at least some of the load of the weight of the turbine wheel from its bearings and tended to reduce frictional problems, as discussed above. However, with a gas, particularly at low velocity, the frictional resistance of paddle wheels rendered them unsuitable to measure gas flow. Also, due to the greatly reduced specific gravity of gases, the buoyant benefits available in a liquid flow meter were not available in a gas flow meter.
U.S. Pat. Nos. 3,788,285 and 3,217,539 disclosed propeller-shaped rotors in flow rate sensors, rather than paddle wheels. Photoelectric circuits were used to detect flow rate based on the rotation of these rotors, as sensed by light reflected off them. Due to their shape, however, these rotors were limited in area of light reflective surface available for use and were further not suited for very low gas flow rate measurements.